Where is the semantic system? A critical review and meta-analysis of 120 functional neuroimaging studies.

Abstract

Semantic memory refers to knowledge about people, objects, actions, relations, self, and culture acquired through experience. The neural systems that store and retrieve this information have been studied for many years, but a consensus regarding their identity has not been reached. Using strict inclusion criteria, we analyzed 120 functional neuroimaging studies focusing on semantic processing. Reliable areas of activation in these studies were identified using the activation likelihood estimate (ALE) technique. These activations formed a distinct, left-lateralized network comprised of 7 regions: posterior inferior parietal lobe, middle temporal gyrus, fusiform and parahippocampal gyri, dorsomedial prefrontal cortex, inferior frontal gyrus, ventromedial prefrontal cortex, and posterior cingulate gyrus. Secondary analyses showed specific subregions of this network associated with knowledge of actions, manipulable artifacts, abstract concepts, and concrete concepts. The cortical regions involved in semantic processing can be grouped into 3 broad categories: posterior multimodal and heteromodal association cortex, heteromodal prefrontal cortex, and medial limbic regions. The expansion of these regions in the human relative to the nonhuman primate brain may explain uniquely human capacities to use language productively, plan, solve problems, and create cultural and technological artifacts, all of which depend on the fluid and efficient retrieval and manipulation of semantic knowledge.

Comparison of the left-hemisphere general semantic network indicated in the present ALE meta-analysis (top) and the “default network” (bottom). The latter map represents brain areas that showed task-induced deactivation during performance of a tone discrimination task, that is, higher BOLD signal during a conscious resting baseline compared with the tone task (see for details). In both types of studies, effects are observed in the AG, posterior cingulate gyrus, DMPFC, VMPFC, ventral temporal lobe, anterior MTG, and ventral IFG. Although effects are stronger in the left hemisphere for both kinds of studies, task-induced deactivation is typically more symmetrical in posterior cingulate and medial prefrontal regions (; ; ; ; ).